Carnitine

Carnitine is a quaternary ammonium compound involved in metabolism in most mammals, plants, and some bacteria.^[1][2][3][4] In support of energy metabolism, carnitine transports long-chain fatty acids from the cytosol into mitochondria to be oxidized for free energy production, and also participates in removing products of metabolism from cells.^[3] Given its key metabolic roles, carnitine is concentrated in tissues like skeletal and cardiac muscle that metabolize fatty acids as an energy source.^[3] Generally individuals, including strict vegetarians, synthesize enough L-carnitine in vivo.^[1]

Carnitine exists as one of two stereoisomers: the two enantiomers d-carnitine (S-(+)-) and l-carnitine (R-(−)-).^[5] Both are biologically active, but only l-carnitine naturally occurs in animals, and d-carnitine is toxic as it inhibits the activity of the l-form.^[6] At room temperature, pure carnitine is a whiteish powder, and a water-soluble zwitterion with relatively low toxicity. Derived from amino acids,^[7] carnitine was first extracted from meat extracts in 1905, leading to its name from Latin, "caro/carnis" or flesh.^[2]

Some individuals with genetic or medical disorders (such as preterm infants) cannot make enough carnitine, requiring dietary supplementation.^[1][3][4] Despite common carnitine supplement consumption among athletes for improved exercise performance or recovery, there is insufficient high-quality clinical evidence to indicate it provides any benefit.^[3][4]